The present invention relates to a cross cutting device and to a method for operating such a cross cutting device.
When individual products, for example individual sheets, are manufactured from an endless flat material web, in particular made of paper, there may be the problem that two successive products cannot be separated from one another by a single cut. This is the case in particular if the position of the edges of a sheet is defined in relation to register marks or water marks and the like. This makes it necessary to cut out portions from the flat material web.
Cross cutting devices having two cross cutting units for manufacturing individual products from a flat material web and for separating cut-out portions from the flat material web or from the product are disclosed, for example, in documents EP-A 0622320, EP-A 1570960, DE-A 3145912, U.S. Pat. No. 5,199,341 and WO-A 95/01245. The cross cutting units each have a fixed blade and a rotating blade arranged on a rotational element, and are spaced apart from one another in the direction of transportation of the flat material web. In a device of the generic type according to EP-A 0622320 or DE-A 3145912 the second rotational element has a zone which can be connected to a suction device and has suction openings and which serves to secure the front edge of the portion to be cut from the flat material web to the second rotational element before the cutting process, and thus to rotate it out of the plane of transportation. The connection to the suction device is interrupted again after the separation of the cut-out portion so that the cut-out portion either drops off under the force of gravity, is scraped off mechanically by the second rotational element or is pushed away with blown air and can be removed. However, the front edge of the following product which is already separated or is still to be separated from the material web should not be deflected out of the plane of transportation but rather be conveyed onward in the direction of transportation. In order to ensure this, suction air is applied to the suction openings in a clocked fashion within a processing cycle.
The processing cycle is understood to mean the processes which lead to the separation of in each case one product and, if appropriate, in each case one cut-out portion, in particular the production of a first separating cut for separating the product and the following material web, and the production of a second separating cut for separating the cut-out portion and the product. The first separating cut is usually carried out by the first cross cutting unit which is located downstream in the direction of transportation, and the second separating cut is carried out by the following, second cross cutting unit. If a cut-out portion is not produced, the second separating cut is dispensed with. Instead of complete separation, it is, for example, also possible to form a perforation.